Airborne conductive contaminants can cause short-circuit failure of electrical and computer equipment. Equipment, such as power supplies, that utilize forced air cooling and have high densities of electrical circuits with high voltages across small node gaps are particularly susceptible. The types of electrically conductive contamination include metallic particulates, whiskers and shards, fragments of wires, and fibres used in anti-static floor coverings. These particulates become entrained in the airflow used to cool the electrical equipment. Metal whiskers are particularly hazardous to electrical equipment because the whiskers are extremely light, having a typical diameter of about 1 micron and a length of about 0.5-5 mm, and are therefore readily entrained in and transported by cooling air flows. These whiskers can grow on surfaces found in computer room environments, e.g. electroplated zinc surfaces, such as present on the undersides of raised floor tiles, inside air conditioning ducts and on the equipment chassis.
The prior art provides two main approaches for the detection of airborne contaminants. According to one prior art method, particulates may be collected on an air filter for subsequent counting, sizing, and identification by compositional analysis. Needless to say, this task is very arduous, cumbersome, and prone to substantial error. Also, since the cumulative number of particulates over some time period is collected and recorded, filters have the disadvantages that time-to-time variations are averaged out and information is not available in real-time.
The other commonly used technique is particle counting. In this technique, particles in a sampled airflow pass between a laser light source and a photodiode which receives the light produced by the laser. Particles in the sampled airflow scatter the laser light. The photodiode detects the interruptions in the light signal and produces electrical pulses. The height of the pulses is directly proportional to the particle size. The pulses are measured and counted by electronic circuitry. The technique allows the number and size distribution of the particulates to be recorded in real-time and enables short term variations in the number of particulates to be captured. However, these types of particle counters detect all the particles over some size range and they are unable to discriminate between different types of particles. Hence, in a dusty environment, a large number of particles will be recorded, but there may be few, or no, electrically conductive particulates, which are of particular concern to gauging the air quality as a source of short circuit failure.